The invention relates to thermoplastic C2-xcex1-olefin copolymer resin blends and flexible films thereof having heat sealing and/or puncture resistance properties. Such blends are useful for making films, particularly heat shrinkable, oriented films for packaging articles and for processing and/or packaging food articles, especially cook-in foods subject to pasteurization processes as well as fresh, frozen, or processed foods such as meat, poultry or cheese.
Manufacturers and wholesalers use flexible thermoplastic packaging films to provide economical, sanitary containers which help protect and/or preserve the freshness and wholesomeness of their products. These films are often sold in bag form. For example, a single or multilayer film is made into bags using a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags. These films and bags may be printed and may also be uniaxially or biaxially oriented, heat shrinkable, irradiated, or may contain film layers which are abuse resistant or puncture resistant or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrougb. Frequently, multilayer films having one or more barrier layers to oxygen and/or moisture such as: saran(a polyvinylidene chloride copolymer); a modified saran e.g. containing methyl acrylate polymer units; ethylene vinyl alcohol copolymer; nylon; or acrylonitrile may be used with a heat sealing layer such as a copolymer of ethylene and vinyl acetate (EVA) to produce bags for packaging oxygen and/or moisture sensitive foods e.g. fresh red meat Such bags help preserve meat in its original condition by preventing or reducing moisture loss and chemical changes in the meat structure due to oxidation reactions.
A typical packaging bag has 1-3 sides heat sealed by the bag manufacturer leaving one open side to allow product insertion. For example, a processor may insert ham, poultry, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other products making a final seal to hermetically enclose the product in the bag. This final seal may follow gas evacuation (i.e. vacuum removal) or replacement of the gaseous environment within the bag by one or more gases to provide some advantage such as to assist product preservation. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer although the actual heat sealing equipment may vary.
Thus, bags are made: by transversely sealing tubular stock of monolayer or multilayer film and cutting off the tube portion containing the sealed end; by making two spaced apart transverse seals on tubular stock and cutting open the side of the tube; by superimposing flat sheets of film and sealing on three sides; or by folding a flat sheet and sealing two sides.
Generally heat seals are made by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the plastic film layers.
A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars of which at least one is heated to cause the layers to fusion bond by application of heat and pressure across the area to be sealed. For example, bags may be made from a tube stock by making one hot bar bottom seal transverse to a tubular film. Once the bottom seal is made, the tube stock is transversely cut to form the mouth of the bag.
After a product is inserted, the bag is typically evacuated and the bag mouth sealed to enclose the product. At one time, the standard method for sealing was to fasten a clip around the mouth of the bag. However, heat sealing techniques are now also commonly employed to produce the final closure of the bag. For example, a bag mouth may be either hot bar sealed or impulse sealed. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name xe2x80x9cimpulsexe2x80x9d), to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are typically cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
Relative to hot bar seals, impulse seals may be made faster because of the quick cool down of the ribbon following the heat impulse. Impulse seals are also generally narrower giving an improved package appearance, but narrower seals also leave less margin for error in the production of continuous sealed edges. Less area is usually bonded in an impulse seal relative to a hot bar seal, thus the performance of the film""s sealing layer is more critical.
Disadvantageously, the film in the seal area often becomes extruded during impulse sealing of known films. This results in thinning of the film and a reduction of film strength in the seal area. In extreme situations, the thinned film is severed or pulled apart. Those skilled in the art refer to severely extruded seals as xe2x80x9cburn throughxe2x80x9d seals. A xe2x80x9cburn throughxe2x80x9d seal does not have adequate strength or integrity to protect the packaged product. One attempt to solve this xe2x80x9cburn troughxe2x80x9d problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a film made from cross-linkable polymer resins causes resin layers in the film to crosslink. Under controlled conditions, crosslinking by irradiation raises and may also broaden the temperature range for heat sealing, and depending upon the film composition may also enhance puncture resistance of the film. If the heat sealing layer of the thermoplastic film is crosslinked too heavily, it is more difficult to fusion bond which makes achieving strong seals difficult, particularly by impulse sealing. All bag seals must maintain their integrity to preserve and protect enclosed products, especially food products.
There must be a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid or solid materials between the bag exterior and interior. This is particularly necessary when the package is made of heat shrinkable film and is to be immersed in hot water to shrink the film against the packaged article since such shrinkage increases the stress on these seals. It is even more critical where the packages are to be immersed at sufficient times and temperatures for pasteurization or cooking. Thus, there is a continuing need for films which can be made into bags having strong seals especially those formed by hot bar and/or impulse sealing. Such films should provide strong seals able to withstand a range of temperatures and also be able to make such seals over a wide sealing temperature range without burn through.
Variations in sealing temperatures, times and pressure are known to exist from one brand or type of sealer to another and also between different sealing machines sold under the same brand. This increases the desirability for films that may be usefully sealed on different sealing machines and over a wide range of temperatures to produce strong integral seals.
Another heat sealing problem is that of inadvertent folding. Normally, a heat seal is made by applying heat and pressure across two portions of film, however, occasionally the area to be sealed will be inadvertently folded to produce a section of film having four or six film portions which are pressed between the opposing sealer bars. In such situations it is desirable to be able to seal the film without burn through. A wider impulse heat sealing temperature range is indicative of a greater latitude in sealing through folds than a narrower range.
Another problem during heat sealing is that of excessively high tear propagation strengths. Lower tear propagation strengths are an advantage in heat sealing operations using impulse sealing technology where the sealing apparatus both seals and cuts the film with the film trim being removed by tearing along the cut. Low tear propagation strengths enable quick trim removal without damage to the seal, film or bag.
A very demanding application for heat shrinkable, heat sealable thermoplastic flexible films is for processing meats. Bacterial contamination during food processing e.g. by Listeria monocytogenes is of great concern. To address health and safety concerns with processed foods, some processors have adopted a surface heat treatment at elevated temperatures sufficient to kill bacteria on already cooked food.
In some demanding applications a food product such as a ham is sealed inside a plastic processing bag or film in which the ham is cooked, refrigerated, shipped and subsequently displayed for retail sale.
In a more common demanding application, food such as a turkey breast, ham, or beef is cooked in a pan, net, or processing film from which the cooked food is removed for further processing such as: slicing; smoking in a smokehouse; treatment with colorants and/or flavorants such as caramel, spices, liquid smoke or honey; glazing; and/or removal of liquid (known as purge) resulting from e.g. the cooking process. Following this further processing, the food product is packaged, often in a printed bag, for shipment and sale. The cooked food is typically placed into a heat sealable, heat shrinkable bag which is then emptied of atmospheric gases by vacuum, heat sealed and subjected to a film shrinking operation usually in a water tank at elevated temperature for a brief period of time to produce a compact attractive package. During these steps which follow cooking and occur prior to packaging for shipment and sale, the food product surface is subject to environmental contamination, for example, by airborne particles, microbes, and dust. The risk from contamination after packaging may be minimized by surface pasteurizing the encased sealed package e.g. in a water bath or steam chamber held at elevated temperatures for a time sufficient to provide the desired degree of protection from microbial contamination and growth. The time and temperatures of this post-cooking pasteurization step may vary widely.
Significantly this surface treatment is in addition to the cooking or pasteurization process and follows hermetically sealing the cooked or pasteurized food in a plastic packaging film. In this demanding use, this xe2x80x9cpost-cooking pasteurizationxe2x80x9d surface treatment is performed after placing the food into the packaging film that will remain on the pasteurized product through sale to an ultimate customer. Often the films are printed with consumer information and brand identification and frequently at least a portion of the film is clear to allow viewing of the enclosed product. Therefore, optical properties and film appearance are important for consumer appeal and sale.
This xe2x80x9cpost-pasteurizationxe2x80x9d film must perform a variety of functions well. It must be puncture resistant and have strong seals at the elevated temperatures encountered in the shrinking operation, and also with the post-cooking pasteurization process. It should also keep tight conformation of the film around the product at refrigeration temperatures with an attractive appearance and act as a good barrier to oxygen, moisture and environmental contaminates.
Various polymers, blends thereof and multilayer films have been employed in attempts to address the above needs and desires of the marketplace. Copolymers of ethylene and vinyl esters such as vinyl acetate have previously been disclosed as useful materials in monolayer and multilayer thermoplastic films and are known for providing heat sealing properties.
An example of a typical fresh red meat bag currently in commerce is a film having three layers which are coextruded and oriented. The core or middle layer of the film is an oxygen and moisture barrier material, the outer layer provides abrasion resistance and is formulated to provide support for the film during the expansion of the primary tube for orientation, and the inner layer provides heat seal properties and contributes to puncture resistance.
The core or barrier layer of this film is a relatively small percentage of total film thickness and is made of polyvinylidene chloridexe2x80x94vinyl chloride copolymer (PVDC or VDC-VC) or vinylidene chloridexe2x80x94methylacrylate copolymer (VDC-MA, or MA-Saran).
The outer layer is thicker than the core layer and is a blend of very low density polyethylene (VLDPE) and EVA. VLDPE, also called ultra low density polyethylene (ULDPE) is a class of ethylene-alpha olefin copolymers having a density ranging from less than 0.915 g/cm3 down to about 0.860 g/cm3, and many commercial VLDPE resins are available having densities from 0.900 up to 0.915 g/cm3. The EVA and VLDPE components contribute to the shrink properties of the film and the VLDPE component contributes to the abrasion and puncture resistance. The VLDPE also adds orientation strength to minimize breaks of the secondary bubble during expansion of the softened primary tube.
By far, the thickest film layer is the inner or heat seal layer. In the above film, this layer is over 60% of the total film thickness and comprises a blend of VLDPE and EVA. The heat seal layer significantly contributes to the puncture resistance of this film. Another desirable characteristic provided by this layer is the heat seal temperature range. It is preferred that the temperature range for heat sealing the film be as broad as possible. This allows greater variation in the operation of the heat sealing equipment relative to a film having a very narrow range. For example, it is desirable for a suitable film to heat seal over a temperature range of 350xc2x0 F. to 550xc2x0 F., providing a heat sealing window of 200xc2x0 F.
Films similar to the general structure and composition as described above have been in commercial use for many years, but efforts continue to be made to increase puncture resistance while maintaining ease of processability, a broad heat seal temperature range and a high degree of both machine direction (MD) and transverse direction (TD) shrink.
Recent developments for improving properties of a heat shrinkable film include U.S. Pat. No. 5,272,016(Ralph). The ""016 Patent improves properties of a multilayer nonoxygen barrier film by use of a blend of EVA, VLDPE and a Plastomer.
U.S. Pat. No. 5,635,261 (Georgelos et al) which application is hereby incorporated by reference, disclose EVA blends useful for their heat sealing properties.
U.S. Pat. No. 5,397,640 (Georgelos et al) discloses a multilayer oxygen barrier film using a three component blend of VLDPE, EVA and a Plastomer. (See e.g. Example 7).
U.S. Pat. No. 5,403,668 (Wilhoit) discloses a multilayer heat shrinkable oxygen barrier film using a four component blend of VLDPE, LLDPE, EVA and Plastomer.
U.S. Pat. No. 5,759,648 (Idlas) discloses a five layer film having a C3C2 heat sealing layer, an EVOH layer, and a VLDPE surface layer connected by special adhesive blend layers. This film is particularly useful in cook-in processing and/or packaging applications.
U.S. Pat. No. 5,928,740 (Wilhoit et al) discloses a flexible film having a blend of an of ethylene alpha-olefin copolymer (EAO) having a melting point (m.p.) between 55 to 75xc2x0 C.; a second EAO having an m.p. between 85 to 110xc2x0 C.; and an unmodified thermoplastic polymer of EAO, LDPE, HDPE, or propylene copolymers, having an m.p. between 115 to 130xc2x0 C. These films may be multilayer, biaxially stretched, heat shrinkable films.
Recent polymer manufacturing changes in catalysts and processes have provided increasing numbers of polymeric resins having different melting characteristics, melting points, and narrower molecular weight distributions (MWD). MWD is the ratio of Mw/Mn where Mw is the weight-average molecular weight of the resin and Mn is the number-average molecular weight. For example, most older EAO and VLDPE resins have a MWD in the range of about 3.5 to 8.0. Improvements in catalysis technology have been able to produce many resins in which this ratio has been reduced to below 3, often in the range of about 1.5 to about 2.5 and most typically about 2.0. A narrower MWD means that the polymer chains of these resins are more uniform in length. A higher MWD resin may be said to comprise polymer chains of more varied lengths. Other changes in resin properties have been attributed to differences in comonomer distribution along an ethylene backbone resulting in materials produced from single-site catalysts having a lower melting point than a multisite catalyst produced polymer of comparable density and melt index. Also, in the case of the above-noted commercial film wherein the heat seal layer is primarily a blend of EVA and VLDPE, it was found that using a more narrow {overscore (M)}w/{overscore (M)}n VLDPE having a lower melting point in place of a broader {overscore (M)}w/{overscore (M)}n VLDPE having a higher melting point considerably decreased the operable heat sealing range. For example, where the sealing layer used only a very narrow {overscore (M)}w/{overscore (M)}n, lower melting point VLDPE in the blend, the heat seal temperature was in the order of 400xc2x0 F. to about 475xc2x0 F. giving a sealing window of only 75xc2x0 F.
Past attempts at providing improved puncture resistance and heat sealing in films, while making some progress, leave much to be desired. Variability in heat sealing equipment and process parameters continue to produce bags with weak seals which are subject to tearing and stress on the seals during cutting operations, which are subject to burn through, which fail to seal through folds, and which produce leaking bags having discontinuous seals and which are not sufficiently resistant to punctures. It would be highly desirable to have biaxially stretched, heat shrinkable films and bags which are highly puncture resistant and/or whose heat sealing layer in particular and film construction in general allows greater flexibility and variability in heat sealing process parameters while producing strong, integral, continuous seals rapidly and with a lower failure rate relative to prior art films and bags.
Accordingly, one object of the present invention is to provide a novel polymeric blend having an improved combination of properties.
It is another object to provide a film of sufficient integrity to withstand the cook-in process with intact seals and film layers.
It is another object to provide a film of sufficient integrity to withstand the post-cooking pasteurization with intact seals and film layers.
Another object is to provide a flexible film having improved heat sealing properties.
Another object is to provide a heat shrinkable biaxially oriented monolayer film having improved puncture resistance and/or lower tear propagation strengths.
Another object is to provide a heat shrinkable biaxially oriented multilayer film having low tear propagation strengths.
Another object is to provide a heat shrinkable biaxially oriented multilayer film having improved puncture resistance.
Another object is to provide a heat shrinkable multilayer film having an improved combination of puncture resistance and low tear propagation strengths.
Yet another object is to provide a heat shrinkable, multilayer film having a puncture resistance and heat sealing range suitable for use in the packaging of fresh bone-in meats.
Yet another object is to provide a heat shrinkable, multilayer film having a combination of hot water puncture resistance and heat seal strengths suitable for use in the pasteurization processing of meats and having low haze and high gloss suitable for retail packaging.
A still further object is to provide a heat shrinkable film having an improved combination of optical and heat sealing properties, and puncture and abrasion resistance.
It is an object of the invention to provide a film for packaging foods such as turkey breasts, beef, or hams which are cooked and shipped in the same film.
It is another object of the invention to provide a process for making a processing or packaging, oxygen barrier, multilayer film having excellent optical properties, strong seals, puncture resistance in hot water and at room temperature, and high shrink values at 90xc2x0 C.
The above and other objects, benefits and advantages of the invention will be apparent from the disclosure below which is exemplary and nonlimiting. It is not necessary that each and every object listed above be found in all embodiments of the invention. It is sufficient that the invention may be usefully employed.
According to the present invention, a novel biaxially stretched, heat shrinkable, thermoplastic, flexible film comprising at least one layer and suitable for use in making bags for packaging e.g. food articles such as primal and subprimal meat cuts is provided. A special inventive blend of at least three copolymers is suitable to being formed into a wide variety of articles including packaging films useful for packaging food and nonfood items alike. In its various embodiments the inventive blend may be used to fabricate inventive films of superior properties and combinations of properties relative to prior art films. These inventive films may have excellent properties relating to tear propagation strength, optics, puncture and abrasion resistance, heat shrinkability, flexibility, heat sealing properties as well as excellent combinations of such properties. Haze values of 10% or less are achievable.
In various embodiments the inventive film comprises a blend including:
(a) a first polymer having a melting point of 80 to 98xc2x0 C., preferably 80-92xc2x0 C., comprising a copolymer of ethylene and hexene-1;
(b) a second polymer having a polymer melting point of 115 to 128xc2x0 C. comprising ethylene and at least one xcex1-olefin; and
(c) a third polymer having a melting point of 60 to 110xc2x0 C. comprising a copolymer of ethylene with an alkyl acrylate or vinyl ester; and optionally
(d) a fourth polymer having a melting point of 80 to 110xc2x0 C. (preferably of 85 to 105xc2x0 C.), preferably selected from the group of ethylene homopolymers such as HDPE and LDPE, and ethylene copolymers with at least one xcex1-olefin.
Various embodiments of the present invention provide a biaxially stretched film having an improved combination of properties e.g. especially high puncture resistance values such as maximum puncture forces of at least 70 Newtons and often at least 120 Newtons or higher, and desirably low tear propagation strength (as measured by the Elmendorf Tear Strength Test) e.g. a tear strength xe2x80x9cxxe2x80x9d such that 10xe2x89xa6xxe2x89xa640 grams per mil in either or each of the machine or transverse directions or x less than 25 grams per mil in at least one of the machine direction (M.D.) or transverse direction (T.D.), without sacrificing high shrinkage at 90xc2x0 C. and other desirable properties. In some embodiments of the invention, films having M.D. and/or T.D. tear strengths of 15 to 25 g/mil (0.59-0.98 g/xcexc) are achieved. Additional embodiments of the invention include films which achieve: a hot water puncture resistance of at least 100 seconds at 95xc2x0 C.; a hot water seal strength of at least 200 seconds at 95xc2x0 C.; a tensile seal strength of at least 400 g/cm at 88xc2x0 C.; a maximum puncture force of at least 70 Newtons, preferably at least 120 Newtons; a shrinkage value at 90xc2x0 C. of at least 40% in at least one direction; a haze value of less than 10%; and/or a gloss value at 45xc2x0 of at least 70 Hunter Units; and preferably combinations of several of these properties.
A preferred four layer embodiment of the invention that is well suited for cook-in or post-cooking pasteurization processing and/or packaging has:
(a) a heat sealing surface layer of at least 50% by weight of (i) a copolymer of propene and at least one xcex1-olefin selected from the group consisting of ethylene, butene-1, methylpentene-1, hexene-1, octene-1 and mixtures thereof having a propene content of at least 60 wt. %, or (ii) at least 50% by weight of a copolymer of ethylene and at least one xcex1-olefin selected from the group consisting of propylene, butene-1, methylpentene-1, hexene-1, octene-1 and mixtures thereof having a melting point of at least 105xc2x0 C. and a density of at least 0.900 g/cm3;
(b) a second polymeric layer having (i) from 5 to 60% of a first copolymer of ethylene and at least one C4-C8 xcex1-olefin, having a melting point of 115xc2x0 C. to 128xc2x0 C., (ii) from 10 to 85% of a second copolymer of ethylene and at least one C3-C8 xcex1-olefin having a melting point of 55 to 98xc2x0 C., and (iii) from 0 to 50% of a third copolymer having a melting point of 60 to 110xc2x0 C. of ethylene with a vinyl ester or alkyl acrylate, wherein the first and second copolymers have a combined weight percentage of at least 50 weight percent, this weight percent being based upon the total weight of the layer;
(c) a third layer having at least 80% by weight (based on the third layer""s weight) of at least one copolymer of vinylidene chloride with from 2 to 20 weight percent (based on said copolymer(s) weight) of vinyl chloride and/or methyl acrylate; and
(d) a fourth polymeric layer having (i) from 5 to 60% of a first copolymer of ethylene and at least one C4-C8 xcex1-olefin, having a melting point of 115xc2x0 C. to 128xc2x0 C., (ii) from 10 to 85% of a second copolymer of ethylene and at least one C3-C8 xcex1-olefin having a melting point of 55 to 95xc2x0 C., and (iii) from 0 to 50% of a third copolymer having a melting point of 60 to 110xc2x0 C. of ethylene with a vinyl ester or alkyl acrylate, wherein the combined weight percentage of the first and second copolymers is at least 50 wt. %, based upon the total weight of this layer; and
wherein the film has an M.D. and/or T.D. shrinkage value at 90xc2x0 C. of at least 40%, and a seal strength of at least 400 g/cm at 88xc2x0 C.
Advantageously, the process of the present invention produces films and bags which are easy to make while having great resistance to puncture and excellent optical properties relative to commercially available prior art films. For example, a process for making biaxially stretched, heat shrinkable film is taught involving the steps of:
(a) extruding a melt plastified primary tube comprising, e.g. the four layer construction described above, or e.g. 20 to 85 weight percent of a first polymer having a melting point of 80 to 98xc2x0 C. comprising at least one copolymer of ethylene and hexene-1;
5 to 35 wt. % of a second polymer having a melting point of 115 to 128xc2x0 C. comprising at least one copolymer of ethylene and at least one xcex1-olefin; and 10 to 50 wt. % of a third polymer having a melting point of 60 to 110xc2x0 C. comprising at least one copolymer of ethylene and a vinyl ester or an alkyl acrylate; wherein the first and second polymers have a combined weight percentage of at least 50 wt. %, the weight percentage being based upon the total weight of the first, second and third polymers;
(b) cooling the primary tube;
(c) reheating the cooled tube to a draw point temperature of 68 to 88xc2x0 C.;
(d) biaxially stretching said tube to a circumference of at least 2xc2xd times the circumference of the primary tube; and
(e) cooling the stretched tube to form a biaxially stretched, heat shrinkable film.